1. Field of the Invention
The present invention relates to a method and system for compensating chromatic dispersion.
2. Description of the Related Art
With a recent increase in the use of a network, a higher capacity is increasingly demanded to the network. At present, a wavelength division multiplexing (WDM) optical transmission system at a bit rate of 10 Gb/s per channel as a base transmission speed is in practical use. It is expected that a higher transmission capacity will become necessary in the future, so that an ultra high-speed transmission system at a bit rate of 40 Gb/s or higher per channel is desired from the viewpoints of frequency efficiency and cost. In such an ultrafast transmission system, waveform degradation due to chromatic dispersion of an optical fiber transmission line must be compensated with high accuracy. The present invention is a technique for optimally performing chromatic dispersion compensation in a WDM optical transmission system.
In an optical transmission system at a bit rate of 10 Gb/s or higher, a chromatic dispersion tolerance is very small. For example, the chromatic dispersion tolerance in a 40-Gb/s NRZ system is 100 ps/nm or less. In the case of a terrestrial transmission system, a repeater spacing is not always constant. For example, in the case of using a 1.3 μm zero-dispersion single-mode fiber (SMF) having a dispersion of about 17 ps/nm/km, the dispersion deviates from the chromatic dispersion tolerance with only a difference of several kilometers in transmission distance.
However, in an optical fiber network owned by a communication carrier, the distance and chromatic dispersion of each repeater spacing are not accurately grasped in the present circumstances. Further, because the chromatic dispersion changes with time according to the temperature, stress, etc. of a fiber, the chromatic dispersion must be accurately measured not only at starting the operation of the system but also during the operation of the system, and the amount of compensation for the dispersion in each repeater spacing must be properly controlled. For example, in the case that a temperature change of 100° C. occurs in a 500-km transmission line using a dispersion shifted fiber (DSF), a chromatic dispersion change becomes about 105 ps/nm as shown below, which value is nearly equal to the chromatic dispersion tolerance of a 40-Gb/s NRZ signal.
(chromatic dispersion change)=(temperature dependence of zero-dispersion wavelength)×(temperature change of the transmission line)×(dispersion slope of the transmission line)×(transmission distance)=(0.03 (nm/° C.)×100 (° C.)×0.07 (ps/nm2/km)×500 (km)=105 ps/nm.
Further, as described later in detail, it is necessary to consider not only chromatic dispersion but also dispersion slope (dispersion tilt or second-order dispersion) in a WDM optical transmission system. This is due to the fact that a plurality of optical signals having different wavelengths are wavelength division multiplexed in a WDM optical transmission system and that chromatic dispersion related to one of the plural optical signals is different from that related to another.